1. Field of the Invention
The present invention relates to an improvement in photovoltaic devices for solar cells and for power sources in various electric appliances. More particularly, the present invention relates to a photovoltaic device (or a photoelectric conversion device) provided with an opaque substrate having a specific irregular surface structure which enhances the utilization efficiency of incident light in the photoelectric conversion region (or the photoactive semiconductor region) and provides improved photoelectric conversion efficiency.
2. Related Background Art
In order to attain a photovoltaic device having an improved photoelectric conversion efficiency, there is known a manner of disposing a metallic reflection layer (which is a so-called metallic back reflection layer) on the side opposite the light incident side of the photoelectric conversion layer. In addition, it is known that in the case of disposing a transparent and electrically conductive layer between a photoelectric conversion layer and a metallic back reflection layer, there can be attained a photovoltaic device having advantages in that the constituents of the metallic back reflector layer are prevented from being diffused into the photoelectric conversion layer, excess current is prevented from flowing, and the photoelectric conversion layer has improved adhesion (see, Japanese Patent Publications Nos. 43101/1984, 41878/1985, and 84888/1985).
Further, there is known a photovoltaic device comprising a transparent and electrically conductive layer composed of TiO.sub.2 interposed between a photoelectric conversion layer and a metallic back reflection layer (see, Y. Hamakawa et al., Appl. Phys. Lett., 43, p. 644 (1983)).
Further in addition, there is known a photovoltaic device comprising a transparent and electrically conductive layer having a textured surface structure provided with a plurality of minute irregularities contacted with a photoelectric conversion layer in which light is scattered at the interface between the transparent and electrically conductive layer and the photoelectric conversion layer to improve the utilization efficiency of incident light in the photoelectric conversion layer (see, H. Deckman et al., Proc. 16.sup.th IEEE Photovoltaic Specialist Conf., p. 1425 (1982)).
However, in the case where a photovoltaic device is intended to operate by adopting a lower electrode (or a back electrode) structure containing the transparent and electrically conductive layer having such textured surface structure, problems with respect to workability and durability are sometimes encountered as will be described in the following.
Herein, it has been generally considered that the conventional so-called textured surface structure in the case of a photovoltaic device is typically of such a configuration with a plurality of pyramid-like irregularities as described in T. Tiedje et al., Proc. 16.sup.th IEEE Photovoltaic Specialist Conf., p. 1423 (1982), and it provides an excellent light-confining effect.
However, in the case of producing a photovoltaic device by providing a substrate having an uneven surface provided with a plurality of pyramid-like irregularities (this uneven surface will be hereinafter referred to as pyramid-like irregular surface) and forming a lower electrode and a photoactive semiconductor layer as a photoelectric conversion layer in this order on said pyramid-like irregular surface of the substrate, a problematic increase in the leakage current sometimes occurs due to a defect or the like in the photoactive semiconductor layer, thereby making it difficult to attain a high yield. Besides this, in this case, since the effective thickness of the photoelectric conversion layer formed above the pyramid-like irregular surface of the substrate is thinner than that of a photoelectric conversion layer formed above a polished surface of a substrate, particularly its constituent doped layer regions (formed at a relatively thin thickness) become further thinned and as a result, the resulting photovoltaic device is eventually such that it is inferior to a photovoltaic device formed on the polished surface of the substrate in terms of Voc (open-circuit voltage) and F.F. (fill factor).
Separately, in the case of a photovoltaic device having a metallic back reflection layer composed of silver (Ag) or copper (Cu), when a positive bias voltage is applied to the metallic back reflection layer under a high humidity environmental condition, the Ag or Cu as the constituent of the metallic back reflection layer can problematically migrate to electrically connect with the electrode (the upper electrode) situated on the light incident side, whereby the photovoltaic device suffers from a shunt (or a short-circuit). This phenomenon is significant in the case where the metallic back reflection layer has an irregular surface (or a textured surface structure) of a magnitude similar to the wavelength of light impinged.
For a photovoltaic device having a metallic back reflection layer composed of aluminum (Al) and having a textured surface, though such migration as occurs in the case of using the metallic back reflection layer composed of Ag or Cu does not occur, the reflectance tends to decrease. Further, in the case where a transparent and electrically conductive layer is stacked on the textured surface of the Al back reflection layer, the reflectance tends to markedly decrease.
Incidentally, for a substrate used in a photovoltaic device, it is known that the surface state of the substrate is closely related to the photovoltaic characteristics of a semiconductor film formed thereon, and as the roughness of the substrate surface increases, the characteristics of the semiconductor film decrease (see, Japanese Journal of Applied Physics, Vol. 21 (1982), Supplement 21-2, pp. 287-290). Therefore, it has been generally recognized that to use a substrate having a surface roughness as small as possible such that is similar to a polished (specular) surface is desired in order to produce a photovoltaic device having desirable photovoltaic characteristics at a high yield.
However, in the case where the surface of each of the substrate and the back reflection layer is made to be a polished surface, the light reflection in the back side is problematically relatively small and because of this, the utilization efficiency of incident light in the photoactive semiconductor layer is insufficient. Depending upon a combination of the material by which the substrate is constituted and that which constitutes the back reflection layer, the adhesion of the substrate with the back reflection layer becomes insufficient to cause layer separation at the interface between the substrate and the back reflection layer during the preparation of a photovoltaic device. Separately, processing a substrate used in a photovoltaic device so as to have a polished surface brings about a rise in the substrate production cost, resulting in making the resulting photovoltaic device costly.
The above problems are certainly possible to occur in the case of adopting a photovoltaic device-producing process which is less costly and adequate in practice by using a low-priced substrate comprising a resin film, stainless steel member or the like or by increasing the photovoltaic device production speed by way of raising the formation speed of a photoactive semiconductor layer, where particularly the yield of a photovoltaic device is decreased.
In order for a transparent and electrically conductive layer used in a photovoltaic device to have such an irregular surface as above described, there is known a manner of making a substrate (on which said transparent and electrically conductive layer is to be formed) having an irregular surface. Particularly in this respect, Japanese Unexamined Patent Publication No. 205879/1991 discloses a solar cell (belonging to a photovoltaic device) in which a glass substrate having an irregular surface is used. However, this manner is problematic in that said irregular surface on the glass substrate is a pyramid-like irregular surface with a random roughness and because of this, those problems above described are liable to occur.
Japanese Unexamined Patent Publication No. 147783/1987 discloses a solar cell (belonging to a photovoltaic device) comprising an amorphous silicon (an a-Si) photoactive semiconductor layer deposited on a metallic substrate whose surface has a specific regular surface capable of causing diffraction of incident light. However, the formation of said specific regular surface on the metallic substrate is conducted by way of the special etching process in this case and because of this, the solar cell is costly.
Thus, there is an increased demand for provision of a highly reliable photovoltaic device in which the utilization efficiency of incident light and the photoelectric conversion efficiency are further improved, which excels in workability and which can be efficiently produced at a reasonable production cost.